High frequency (HF) welding is useful in flexible packaging, flexible bag production, textile lamination, and in producing automotive components such as headliners and sunvisors. HF welding is an alternative to heat-bonding methods for adhering a film to a substrate such as the film itself, another film, or a textile fabric. HF welding involves heating only a HF-active component or HF-active layer of a structure such as a multilayer film sufficiently to soften that component. The selective heating is accomplished by treatment with high frequency radiation. In contrast, heat-bonding methods require transferring heat through an entire structure to soften a bonding layer or component in the structure. In each case, the softened layer or component subsequently bonds the film structure to a substrate.
HF welding can be advantageous over heat-bonding methods. First, HF welding can bond a film in a fraction of the time required for heat-bonding methods. Second, HF welding is less likely to degrade thermally sensitive material, such as oriented films and thermally sensitive dyes. Third, bonding complex shapes is possible using HF welding.
Flexible polyvinyl chloride (f-PVC) has been used in HF-active films due to its HF sealing capability, vapor and gas barrier properties, and flexibility. Films of f-PVC typically include a plasticizer, typically a phthalate plasticizer, to enhance film flexibility. The plasticizer can migrate out of the polymer over time, decreasing film flexibility and potentially contaminating materials in contact with the film. A desire for long-term flexibility and concern about the environmental impact of halogenated polymers, such as f-PVC, make it desirable to have another HF-active polymer as an alternative to f-PVC.
Olefin/acrylate copolymers and olefin/vinyl ester copolymers demonstrate HF activity when they contain greater than about 12 weight percent of a polar comonomer such as alkyl acrylate or vinyl ester. Such a high level of polar comonomer reduces a polymer's crystalline melting point (Tm) below 100° C., and generally below 90° C. Polymers having such a low Tm are not suitable for use in many articles where a film must maintain physical integrity through multiple exposures to temperatures around, and particularly above, 100° C. Examples of such articles include many textiles, such as clothing articles, which are subject to repeated washing and drying cycles. Additional examples of such articles include sun visors for automotive interiors. Olefin/acrylate and olefin/vinyl ester copolymers also tend to have a lower dielectric loss factor (DLF) than f-PVC. A lower DLF means more HF energy is necessary to weld the copolymers with HF than needed for f-PVC. Addition of HF-active fillers can help increase the DLF of a copolymer film, but may do so at the expense of physical properties such as tensile strength.
Commercially cost effective HF-active film-forming polymer compositions and HF-active films that have long-term flexibility and Tm greater than 90° C. are desirable as alternatives to f-PVC compositions and films. Preferably, the polymer compositions and films are also essentially halogen-free.
PCT Patent Application publication WO2002/102898 discloses a blend of 20 to 80 percent, by weight, of a low weight-average molecular weight copolyester with a carboxyl-containing polyolefin that has a dielectric loss (DLF) factor of 0.05 or more at 27 MHz and 23° C. This composition is reported to be HF-active.
It can be desirable to replace f-PVC with a HF-weldable composition that can remain flexible without the inclusion of a plasticizer, and which is compatible with the environment.